1. Field of the Invention
The invention relates in general to refractory metal compacts and their preparation, and, in particular, to the inexpensive low temperature sintering of refractory metal compacts that are of a desired density, up to and including substantially fully dense, and exhibit high refractory grain contiguity.
2. Description of the Prior Art
The low temperature sintering of refractory metal compacts had generally been considered to be impossible or impractical. It is well recognized in the art that various methods of sintering are available to form sintered metal compacts from powdered materials. Some of these prior methods had generally been considered to be unsuitable for use with refractory metals. Some problems that were typically encountered in processing refractory metals included excessive grain growth, low contiguity, low melting matrix phases, an inability to achieve substantially full theoretical densities, and high processing costs.
Liquid phase sintering, according to the teachings of the conventional prior art, generally involved selecting a base metal-additive combination wherein the additive exhibited a high solubility for the base metal, but a low solubility in the base metal. A liquid base metal-additive phase formed at a liquid phase sintering temperature below the melting point of the base metal. Sufficient additive was provided to form a liquid phase consisting of additive and base metal. Because the additive did not dissolve to any significant degree into the base metal, the liquid phase was permanent. Upon cooling the resulting prior art compacts generally contained a solid base metal phase and a solid phase at the grain boundaries that corresponded to the composition of the liquid phase. The production of two or more solid phases was generally an unavoidable artifact of the prior art liquid phase sintering processes. The presence of such a low melting, and usually low strength, solidified liquid phase (matrix or binder) is generally undesirable in most refractory metal compacts.
The sintering of refractory metals was generally believed to inevitably result in substantial grain growth. Grain growth is a function of surface area. The amount of grain growth increases with increasing surface area, and surface area increases as the average diameter of the particle decreases. Thus, the smaller the grain, the more grain growth will occur. With many prior refractory metal sintering processes it did not matter much what the initial grain size was, the grains would grow to about the same size in the finished compact. Grain growth is undesirable, because the properties of the compact generally improve as the grain size decreases.
Activated solid phase sintering, according to the teachings of the conventional prior art, generally involved selecting a base metal-additive combination wherein the additive exhibited a high solubility for the base metal, but a low solubility in the base metal. In this respect the prior activated solid phase sintering process was similar to the prior liquid phase sintering process. The quantity of the additive, however, was generally insufficient to form a significant liquid phase, for example, less than about 0.3 percent by weight of the total mixture. The additive remained at the interfaces between the refractory metal grains where it served to activate the sintering operation. Upon cooling the additive was still present at the grain interfaces. Because the additive was selected to be substantially insoluble in the base metal, it could not migrate into the base metal. Grain contiguity was thus seriously impaired. For most refractory metal applications this resulted in degraded physical properties as compared to compacts with high grain contiguity. Activated sintering generally proceeded more slowly than liquid phase sintering. During processing the compact was held at elevated temperatures for a considerable period of time. With refractory metals this generally resulted in substantial grain growth. Also, additional processing steps, such as pressurizing the compact, and the like, were generally required to achieve substantially full density. These additional processing steps complicated the process and increased the expense.
Transient liquid phase sintering, according to the teachings of the conventional prior art, generally involved selecting a base metal-additive combination wherein the additive formed a liquid with the base metal, and also exhibited a high solid solubility in the base metal. During the process of sintering, a liquid phase was formed. This liquid phase then rather quickly disappeared and liquid aided sintering ceased, because the additive dissolved into the solid phase base metal. The liquid phase did not solidify into a second matrix phase, rather, it simply disappeared as the additive dissolved into and alloyed with the base metal. When the system cooled there was no detectable solid phase corresponding to the liquid phase. High grain contiguity was generally achieved. With refractory metals it had generally been considered impossible to achieve substantially full theoretical density with transient liquid phase sintering alone. A high temperature and/or high pressure step was generally believed necessary to achieve full density. Final densification required solid state diffusion at temperatures close to the melting point of the refractory metal for long periods of time. The properties of the final refractory metal compact were primarily determined by the high temperature conditions in the final densification step. Grain growth was generally limited in transient liquid phase sintering operations because the compact was held at the sintering temperature for only a relatively short period of time. Unfortunately, substantial grain growth occurred in the final high temperature, densification step, so no significant advantages were realized from the use of transient liquid phase sintering with refractory metals.
Previously it had been believed that it was not possible to achieve both transient liquid sintering and activated sintering with the same system. Transient liquid sintering was generally believed to require an additive with high solubility in the base metal. Activated sintering, by contrast, was known to require an additive with low solubility in the base metal. These two requirements were previously believed to be incompatible with one another.
Applicant has discovered how to hold an effective amount of a metallic reagent at the boundaries between refractory metal grains long enough for the metallic reagent to activate the sintering of the refractory metal grains in a compact. As the compact reaches substantially full density the metallic reagent diffuses away from the boundaries into the refractory grains.
These and other difficulties of the prior art have been overcome according to the present invention.